The invention relates to an optical fibre grating sensor system and to a method for increasing the number of measurement sites in such a system. In particular, the invention relates to a system and method using Fibre Bragg (FBG) or Long Period Fibre (LPG) Gratings, and contemplates the use of the system applied to a wind turbine power plant.
Wind turbine components are subject to temperature variation from a number of sources, such as environmental temperature change, and heat generated in components. Consequently, it is important that the temperature of those components is monitored to ensure that they are operating within appropriate ranges.
In addition, wind turbine components are subject to deformation or strain from a number of sources, such as the accumulation of particulates like dirt or ice, their own weight, and the force exerted by the wind itself. Consequently, it is important that the strain on components be monitored to ensure that they remain fit to operate over their intended working lives.
An FBG sensor is an optical fibre in which an optical grating is formed. The grating itself is typically a periodic variation in the refractive index of the fibre, tuned to reflect a particular wavelength of light. The region of the optical fibre having the grating is then attached to the region of the wind turbine component where an operating condition such as temperature or strain is to be measured. It is attached in such a way that any deformation, strain or temperature change experienced by the component is transmitted to the fibre and to the grating. Temperature variation, deformation and strain causes the spacing and the refractive index of the grating to change, and causes a detectable change in the wavelength of light reflected back or transmitted by the grating. Various arrangements are known for inserting light into the FBG sensors and for extracting and analysing the output.
Long Period Fibre Gratings operate in a similar manner to FBGs but instead couple light of particular wavelengths travelling in the core to the cladding where it is subsequently lost due to absorption or scattering.
A number of techniques for mounting fibre optic sensors on wind turbine components are known, such as attaching the fibre optic cable by means of brackets, or hollow casings, or locating the sensor within a capillary tube that can be embedded, tube and all, in a composite material. When attaching a fibre optic sensor, it is important that the sensor will not be damaged by the mounting means, either when the cable is mounted or later during the operational life of the sensor. However, for strain sensors, it is also important that the sensor be sufficiently sensitive to strain on the component.
Present FBG sensors pose a number of disadvantages. In particular, in order to measure temperature, or strain, at a number of different locations a series of FBGs need to be used, each FBG being tuned to a different wavelength. In addition, the range of wavelengths that each FBG operates over need to be distinct from each other FBG's range of operating wavelengths. This is illustrated by way of example in FIG. 8. This is necessary to enable a single wavelength to relate to a specific FBG and a specific operating condition. Such systems require a broad band light source and a broad band interrogator, or a narrow band tunable light source, which results in significant overall cost of the system. LPGs suffer from similar problems.
We have appreciated that there is a need for a more cost effective solution for detecting temperature changes and/or deformation at multiple locations in a wind turbine component.